Plasma-chemical processes are used, for example, for making solar cells, with formation meaning both deposition of a layer and also "stripping" and treatment in the course of a chemical reaction in a plasma discharge. The latter is known from German patent specification DE-OS 21 52 895, for example, in which the efficiency of a thin-layer solar cell of the copper sulfide/cadmium sulfide type is increased by applying a further copper-containing layer to the copper sulphide layer, said copper-containing layer being thin compared to the copper sulphide layer. The copper content can be generated in the additional layer by reduction of the copper sulphide layer on the surface, by means of treatment in the glow field in a hydrogen atmosphere. The deposition of layers to make functional solar cells is known for example in connection with inversion-layer solar cells. Glow processes are also used, for example, for cleaning reaction chambers used to make a-Si solar cells. Here, the glow discharge is used for cleaning by etching away polysilane or polymers of the doping gases, for example, present in the reaction chamber.
In plasma-chemical processes for depositing amorphous silicon layers, for example, conventional electrode geometries are used. This means that two opposite electrode plates are used. Although this results in a homogeneous electrical field, there are problems with the gas supply if the gas is fed in from the side. This drawback applies particularly when large surfaces have to be coated. At high deposition rates, the gas mixture is depleted of SiH.sub.4 and doping gases such as B.sub.2 H.sub.6 or PH.sub.3, with the result that the layer thickness or the doping is irregular.
A further drawback of an electrode arrangement of this type is that the quality of the layer is affected during its growth by bombardment with charged particles, i.e. ions or electrons.
In this connection, an arrangement is known for the deposition of amorphous silicon layers in which the substrate surface to be coated is not arranged between two opposite plate electrodes but laterally outside the platesd, with the intention that the forming electrical field if possible runs parallel to the substrate surface.
A drawback of an arrangement of this type is however that by the very nature of the plate capacitor arrangement a homogeneous field can only be achieved between the plates and when said plates are close enough together, which rules out per se the coating of large surfaces with field components parallel to the substrate surface. Outside the plate capacitor, the field is inhomogeneous--particularly at the edges--and also very weak. Coating of large-area substrates in this electrode geometry is therefore difficult and ineffective (low silane yield), because firstly the inhomogeneous field distribution causes an uneven layer thickness and secondly the deposition rate becomes very small as the distance between the plates grows.
The problems of this arrangement are therefore that the fundamental consideration to generate the parallel electrical field between two plates is no longer correct when the plates are widely spaced (distance&lt;&lt;plate diameter), i.e. is no longer suitable for coating large substrates (approx. 100 cm.sup.2 or more).
To improve the homogenization of the deposition rate, a third electrode is introduced as an auxiliary electrode which runs parallel to the electrical field. For large-area coating, this electrode system is repeated row by row such that an arrangement of the electrodes with alternating polarity is provided.
FIG. 1a illustrates a system of electrodes with alternating polarity which produces an inhomogeneous electric field. The system includes electrodes (10-20) whose polarity changes from one electrode to the next from +U to -U, to +U to -U, and so on.
Considering the first dipole given by the electrodes 10, 12, the broken lines show the electric field which is directed from the plus pole to the minus pole. The next dipole given by the electrodes 12, 14, has the same electric field line density, but the field vector has an opposite direction. All the following dipoles are operated in the same manner. In FIG. 1a, only one line of the dipole filed is drawn as a broken line.
The resulting electric field distribution can be found by superposition of the components of all dipole fields, respectively. However, only the contribution of the nearest neighbors has been considered. This is nearly correct because the contributions of the electric field strength of more remote dipoles are small enough to be neglected. By adding the vector components of the electric field in every point, a field distribution results which is drawn by continuous lines in FIG. 1a. The resulting field alternates and is concentrated near the electrode rods (10-20); it is directed vertically to the plane in which the rods are located. Between the rods, the field is weak and directed parallel to the plane in which the rods are located. Therefore, an alternating electrode polarity results in an inhomogeneous electric field which is not well suited for plasma deposition of homogeneous layers.
The electrode configuration as described can be considered as a boundary case for the plate condensator arrangement, too, if the plates are widely spaced (distance &lt;&lt;plate diameter).
A method of the type illustrated in FIG. 1a can be found in U.S. Pat. No. 4,399,014, where a large number of rod-like electrodes are spaced equally. The polarity of the electrodes is also selected to be alternating. The substrate surface to be treated is then between two electrode systems. This method is used commercially for surface glow treatment of plastic PCBs. The heavy bombardment of the surface with ions that is achieved with this arrangement is a consequence of the direction of the accelerating electrical field vertical to the substrate surface. As a result of the particle bombardment the surface is cleaned and better adhesion conditions are obtained by breaking up of chemical bonds. In the case of layer manufacture, however, this effect is undesirable; the reason is that when the electrical field runs vertical to the substrate surface, so that ions and electrons can directly hit the growing layer, a deterioration of the substrate surface occurs, so that a manufacturing method of this type is disadvantageous particularly for thin-layer solar cells such as a-Si cells. Furthermore, the electrical field is inhomogeneous because of its structure (see FIG. 1a), so that this too entails a negative effect on the deposition rate.